The lateralization of two tone bursts presented separately to the left and right ears will be measured as a function of their onset time difference and of their difference in frequency. A forced-choice paradigm will be used; in effect the subject will judge which ear is stimulated first. The frequency separation between the two tones will be varied parametrically in different regions of the audible frequency range in order to determine how lateralization depends on frequency separation and whether the critical band plays a role in lateralization. Similar measurements will also be made as a function of loudness level and of stimulus duration to determine whether the width of the critical band changes with level or duration. Measurement will also be made of the recognition of speech presented to one ear though a filter centered on a high frequency and to the other through a filter centered on a low frequency, as in the Matzker binaural hearing test. These measurements will be correlated with the subjects' ability to fuse dichotic pure tone that differences in the fusion of pure tones and their relation to the integration of dichotic speech signals. The relative importance of interaural loudness an intensity differences in the lateralization of pure tones will be assessed in normal listeners both in the quiet and against a unilateral masking noise, and in hard-of- hearing listeners with large asymmetries in sensitivity between the two ears. Special attention will be paid to adaptation in these measurements.